In packet switching systems, customer interface units connect customer terminals to the packet switching system via modulator demodulator circuits (MODEM) or direct access circuits. The packet switching system directs the operation of the customer interface units by transmitting selection and control information, and the customer interface units inform the packet switching system of their status by returning status information.
A problem in such prior art systems arises when an improperly functioning customer interface unit responds to selection signals intended for another customer unit and stores incorrect control information. The stored incorrect information drives the faulty customer interface unit into a variety of incorrect states, obviously causing unreliable customer service.
In the prior art, erroneous response by an improperly functioning customer interface unit to the transmission of control information could be detected by requiring each customer interface unit to insert its own device number into the status information before transmission of this information to the packet switching system. However, this method increased the cost of the customer interface unit and required that the packet switching system periodically poll each customer interface unit to ascertain that it is functioning correctly.
Erroneous transmissions by a customer interface unit could also be detected by dedicating specific detection circuitry in the packet switching system to each customer interface unit for detecting erroneous transmissions. This arrangement results in degradation of packet switching system reliability due to the additional circuitry required to perform the monitoring on a per customer interface basis.